The present invention relates to protective apparatus of a vehicle microcomputer and, more particularly, to protective apparatus for preventing an abnormal vehicle control operation by the microcomputer.
Japanese Patent Publication No. 62-12536 discloses a known technique for protective apparatus of a microcomputer. The apparatus disclosed in that Japanese Patent Publication uses an output signal of a watch dog circuit as a reset signal for a computer. The level of the output signal from the watch dog circuit is inverted when an input signal pulse is not detected for a time interval that exceeds a predetermined time interval. Apparatus for overcoming a hardware abnormality of the computer system comprises circuit means for inhibiting the computer from accessing a memory and for outputting a signal which indicates a hardware abnormality when the output signal is repeated a predetermined number of times. The protective measure against program malfunction is accomplished by using the hardware abnormality indicating signal.
Accordingly, when program malfunction occurs, a reset pulse is supplied to the computer. In addition, the number of successively generated reset pulses is counted. When the counted number exceeds a predetermined number of reset pulses, it is determined that a hardware abnormality has occurred. When such a determination is made, access to the memory is inhibited in order to protect data, while an indicating unit indicates the abnormality to an operator. As a result of these operations, memory destruction due to hardware abnormalities can be minimized.
In accordance with the above described hardware abnormality protective apparatus of the computer system, a predetermined number of computer reset signals, which are output signals from the watch dog circuit, must be counted. However, before the predetermined number of signals from the watch dog circuit are counted, the processing results of signals output from the computer system are indeterminate. As a result, a device driven by the computer system may be abnormally operated while the output signals from the watch dog circuit are being counted.
For example, assuming that a microcomputer comprising a vehicle electric power steering system causes such an abnormal operation, an output not desired by the vehicle driver may be generated.